A226137 - OEIS

%I #10 Jan 18 2022 03:36:47

%S 1,2,3,4,6,7,10,14,15,22,32,46,47,69,101,147,148,217,318,465,466,683,

%T 1001,1466,1467,2150,3151,4617,4618,6768,9919,14536,14537,21305,31224,

%U 45760,45761,67066,98290,144050,144051,211117,309407,453457,453458

%N Positions of the integers in the ordering of rational numbers as generated by the rules: 1 is in S, and if nonzero x is in S, then x+1 and -1/x are in S.  (See Comments.)

%C Let S be the set of numbers defined by these rules:  1 is in S, and if nonzero x is in S, then x + 1 and -1/x are in S.  Then S is the set of all rational numbers, produced in generations as follows: g(1) = (1), g(2) = (2, -1), g(3) = (3, -1/2, 0), g(4) = (4, -1/3, 1/2), ... For n > 4, once g(n-1) = (c(1), ..., c(z)) is defined, g(n) is formed from the vector (c(1)+1, -1/c(1), c(2)+1, -1/c(2), ..., c(z)+1, -1/c(z)) by deleting previously generated elements.  Let S' denote the sequence formed by concatenating the generations.

%C A226130:  Denominators of terms of S'

%C A226131:  Numerators of terms of S'

%C A226136:  Positions of positive integers in S'

%C A226137:  Positions of integers in S'

%H Clark Kimberling, <a href="/A226137/b226137.txt">Table of n, a(n) for n = 1..48</a>

%e S'= (1/1, 2/1, -1/1, 3/1, -1/2, 0/1, 4/1, -1/3, 1/2, ...), with integers appearing in positions 1,2,3,4,6,7,...

%t g[1] := {1}; z = 20; g[n_] := g[n] = DeleteCases[Flatten[Transpose[{# + 1, -1/#}]]&[DeleteCases[g[n - 1], 0]], Apply[Alternatives, Flatten[Map[g, Range[n - 1]]]]]; Flatten[Map[g, Range[7]]]  (* ordered rationals *)

%t Map[g, Range[z]]; Table[Length[g[i]], {i, 1, z}] (* cf A003410 *)

%t f = Flatten[Map[g, Range[z]]];

%t Take[Denominator[f], 100] (* A226130 *)

%t Take[Numerator[f], 100]   (* A226131 *)

%t p1 = Flatten[Table[Position[f, n], {n, 1, z}]] (* A226136 *)

%t p2 = Flatten[Table[Position[f, -n], {n, 0, z}]];

%t Union[p1, p2]  (* A226137 *)  (* _Peter J. C. Moses_, May 26 2013 *)

%Y Cf. A226080 (rabbit ordering of positive rationals).

%K nonn

%O 1,2

%A _Clark Kimberling_, May 28 2013